1. Field of the Invention
The present invention relates to a procedure for setting up an emergency call in a wireless local loop.
2. Description of the Related Art
The invention concerns a wireless local loop system, in which a subscriber is connected over a wireless link via an access node (AN) to a wired network local exchange. The subscriber terminal devices are connected to the access node via a GSM/DCS 1800 interface (GSM, Global System for Mobile Communications; DCS, Digital Cellular System) . The access node is connected to the local exchange via a V5.2 interface consistent with the ETS 300 347 standard series. GSM is a European digital mobile communication system standardized by ETSI. DCS-1800 is a mobile communication system standardized by ETSI, which is based on the GSM specification and aims at a more effective use of microcells and which works in the frequency range of 1800 MHz. Between the terminal device and the access node there is a base station, through which call signals sent by the terminal device over a radio channel are transmitted via the access node to a public telephone network and vice versa. The access node can be connected to the telephone exchange using e.g. the V5.1 or V5.2 protocol.
Open interfaces (V5.1 and V5.2) between an access node and a telephone exchange are defined in the ETSI (European Telecommunications and Standards Institute) standards of the ETS 300 324 and ETS 300 347 series. V5 interfaces enable subscribers belonging to a physically separate access network, which may be either a wired or a wireless network, to be connected to a telephone exchange using a standard interface. A dynamic V5.2 concentrator interface consistent with the standards ETS 300 347-1 and 347-2 consists of one or more (1-16) PCM (Pulse Code Modulation) cables. One PCM cable comprises 32 channels, each of which with a transfer rate of 64 kbit/s, i.e. 2048 kbit/s in all. The V5.2 interface supports analogue telephones as used in the public telephone network, digital, such as ISDN (Integrated Services Digital Network) basic and system subscriptions as well as other analogue or digital terminal equipment based on semi-fixed connections.
A call initiated with an EMERGENCY_SETUP message as described in the GSM system specification may, if the resources prove to be insufficient, star a process of forced release of a call having a lower priority.
When a subscriber connected to the access node of a wireless local loop tries to make an emergency call to a public emergency number and if the call control system of the local exchange is unable to find a free time slot in the V5.2 interface within a predetermined period of time, then the call setup will fail. The subscriber will keep trying again until the call is successfully set up. The standard V5.2 interface contains no definition of forced call setdown or an emergency call as such. When the setup of an emergency call fails due to congestion, the WLL terminal will generate a busy tone for the subscriber. Consequently, the subscriber will try to call again. In practice, the entire signalling required for call setup is generated anew, starting from the allocation of a communication channel, resulting in unnecessary burdening of the wireless network resources.